The direct bonding technique represents a relatively new development in orthodontics. It Offers several advantages over the older banding technique, which includes, among others, improved esthetics, greater patient comfort, reduction in the incidence of tooth decalcification, elimination, in some cases, of the necessity of extracting teeth, chair-time savings and facilitation of the bonding to partially erupted teeth. The adhesives used for bonding orthodontic brackets, plastic or metal, to human teeth differ in their form and application techniques. The direct bonding adhesives which are currently being used, belong to one of the following categories. 1. A four-component system comprised of two liquids which cure when mixed together and are applied on acid etched teeth as a primer and two pastes which cure when mixed together and are applied over the primer to bond the bracket.
2. A simplified system comprised of only two pastes which cure upon mixing; such a paste being of a lighter consistency than the paste described in Point 1 and therefore, not requiring priming.
3. A powder and liquid system which cures upon mixing together. Usually such a mix is thin enough to allow for the elimination of the use of the primer.
4. A two-component system representing two light-consistency pastes or a liquid and a paste; one of such components is applied to the tooth and the other on the bracket. Such a system cures when the bracket is pressed against the tooth and, therefore, both components come into contact.
5. A UV-cured system comprises of one or two components (a liquid primer and a paste or a light-consistency paste only) which cures upon exposure to light with a wave-length of 300-400 nm. Such a system is suitable for use with transparent plastic brackets or with metal brackets having perforated bases only.
The first four types described above belong to the self-curing category and they share common disadvantages such as limited working time, limited shelf-life, difficulties in cleaning off the excess material around the bracket periphery and susceptibility to operator error in mixing proportions, misjudegment of working time, etc. which may result in adhesive failures. The UV-cured adhesives suffered severe limitations as their use was limited to the type of brackets that are inadequate in the majority of applications (plastic brackets) or to the brackets of an obsolete style and inferior in performance (metal brackets with perforated bases).
While the adhesives of the prior art are used with success, their limitations and shortcomings are well recognized. There is a recognized need for an one-component adhesive which is thermally-stable and universal in application when it comes to the design of the bracket used, clinical situations and adaptability to various techniques. Such adhesive should cure only after it is activated and should reach its maximum strength in a very short time. Therefore, such an adhesive would allow the operator to correct the bracket position whenever and wherever desirable, to bond to either the buccal or lingual tooth surfaces, to easily remove excess material and also would allow for substantial chair-time savings.